Response characteristics of gob-side roadways with pre-split thick hard roofs in extra-thick coal seams: based on strain-softening effects

Pre-splitting of the thick, hard roof improved the stress state of gob-side roadways and reduced the degree of plastic failure. Based on the strain softening effect, methods for calculating the plastic softening zone (SZ) and residual zone (RZ) widths of the goaf coal wall in extra-thick coal seams were derived. It revealed the relationship between the plastic failure characteristics and the lateral support stress concentration coefficient K, and solved the evolution laws of the stress and displacement fields during different plastic failure stages. Results indicated that: (1) The width of RZ is linearly correlated with the value of K and coal seam burial depth (H), whereas the width of SZ shows no clear correlation with K and H; (2) The vertical stress and shear stress at the roof interface remain low in the RZ but increase in a curved trend in the SZ. The horizontal stress in the middle of the coal seam increases linearly in the RZ and follows a curved rise in the SZ. The horizontal displacement shows a maximum value at the coal wall surface, but becomes smaller deeper into the coal mass; (3) After pre-splitting the thick hard roof, the PZ range significantly decreases, the lateral support stress concentration coefficient decreases substantially, and the stress state of goaf coal wall is substantially improved. This provided a favorable prerequisite for gob-side roadways driving and maintenance during the mining of the next section.